Electrodeposited tin and process of electrodeposition



Jan. 27, 1942. M. SCHLOTTER 2,271,209

ELECTRODEPOSITED TIN AND PROCESS OF ELECTRODEPOSITION Filed Oct. 28, 1937 CuERE/VT 500 ,400 500 600 Overvo/foye /'/7 /77////VOLI3 in /uence q G'e/af/ne 0N0 fies/n an pverm/fuye l'n depQJ/T/Qn 9/770 INVENTOR. MXJCHLTTEE.

ATTORNEY Patented Jan. 27, 1942 ELECTRODEPOSITED TIN AND PROCESS OF ELECTRODEPOSITION Max Schliitter,

mesne assignments, of one-half States Steel Corporation, New York, N. Y., a

Berlin, Germany, assignor, by

to United corporation of New Jersey, and one-hall to Carnegie-Illinois Steel Corporation,Pittsburgh,, Pa., a corporation of New Jersey Application October 28, 1937, Serial No. 171,420 In Germany June 5, 1934 Claims.

This application is a continuation in part of my pending application'Serial No. 25,000 filed June 5, 1935.

My invention relates to the electrolytic deposition of metals, particularly tin, in bright, glossy deposits.

Heretofore acid solutions of metal salts free irom'alkali and ammonium have been proposed for the electrodeposition of metals, such as tin, and the addition of colloidal substances or substances in colloidal condition to the metal bath has been suggested. None of these suggested or proposed processes has, however, led to or resulted in the production of bright deposits directly, but only to products which are coarsely granular or havea cloudy or clouded appearance.

My present invention provides an electrodeposit of tin and other metals which is in such form as deposited as to give a bright and glossy appearance and provides, furthermore, a process whereby bright and glossy deposits of tin and other metals suitable produced by electrolysis of acid metal baths.

In my present invention electrolysis of a solution of a metal salt, for instance a tin salt, preferably a stannous salt, in an acid containing aqueous medium is carried on under a greatly increased hydrogen overoltage in the presence of a mixture of colloids, or of materials in colloidal solution or state in said aqueous medium, at least one of the colloids being lyophile or hyfor electroplating may be drophile, or electro-negative, and at least one being a lyophobe or hydrophobe, or electro-posltive, to the solution and in which the electronegative colloidal dispersed material is adsorbed or otherwise associated with the electropositive material, so that both migrate to the cathode. Otherwise expressed, one of the materials in colloidal dispersion in the acid tin or other metal bath would normally migrate to the anode, and the other one would normally or, if alone in the acid metal bath, migrate to the cathode. But when these two colloidally dispersed materials are both present in the electrolytic bath of my invention, they migrate to'the cathode by electro-osmosis, thus indicating that both of the colloids are associated in a larger dispersed particle containing both electro-negative and electro-positive dispersed particles, and having a residual positive charge. Some of the metal ions. may also be adsorbed in this larger colloidal aggregate of positive and negative or hydrophobe and hydrophile particles. It will be understood that these colloids may be present in very small amounts. Preferably, they are of organic nonconducting materials or substances;

The combination of colloids thus obtained in my electrolytic bath has the effect of greatly increasing the hydrogen over-voltage over that which could be obtained by using either of the colloidally dispersed materials alone, even in an amount equal to the total of both colloids. This increase in over-voltage'permits the production of an electro-deposition of novel structure that is highly advantageous.

In the accompanying a graph showing the relation between overvoltage in milli-volts as abscissae and corresponding current densities as ordinates for tin baths of identical composition except'that some contain resins in different proportions, others gelatin and others a mixture of gelatin and resin,

As shown in the drawing, for example, 0.4% of resin increases the overvoltage somewhat. The presence of 0.2% of resin and 0.2% of gelatin increases the over-voltage much more, particularly at the lower current densities. The increase over the over-voltage obtained with gelatin alone is even greater.

In the electrolysis of acid tin baths according.

to my invention, therefore, to an acidified aqueous stannous solution,- there are added at least two different colloids, one being a hydrophobic colloid electro-positive to the solution, and. the other a hydrophilic colloid electro-negative to the solution, the two colloids being organic inaterials and not mutually precipitated, nor precipitated by the tin or acid of the solution, so

as' to form a substantially permanent colloidal I solution. The two opposite colloidal materials are so chosen that the negatively charged particles will be adsorbed to, or will aggregate with, the positively charged materials so that both will migrate to the cathode.

Electrolysis is then carried on through this bath by the passage of current from a tin anode to the material to be plated at the cathode. The current density will be sufficient to provide a high hydrogen over-voltage. sity is about at the point, or slightly above the point, at which hydrogen is discharged at the cathode. In other words, the current, density in each case is increased until minute bubbles of hydrogen appear.

A large number of substances may be employed in conjunction to form a hydrophile,.or

positively charged, and a hydrophobe, or negatively charged, dispersion. Examples of hydrodrawing the figure is This current den-' phile or lyophile colloids are glue, gelatin, agaragar and albuminous substances. Examples of By electrical charge is meant the charge of the particle in the particular electrolytic bath solution although it may have a diilerent charge in another medium, an alkaline one, for example. An instance is the dispersed rubber of latex, which is negative when the dispersing medium is alkaline and positive when the dispersing medium is acid. Also the composite dispersed particles will have a residual positive charge and move to the cathode where they will cause a greatly increased hydrogen overvoltage as compared with either of the charged particles alone. It will be also understood that the. term phenol includes all aromatic compounds having the phenol radical and that glue includes gelatin, albuminous substances, agar agar and similar gelatinous materials.

It will be understood that the above combinations are given merely by way of example and that the various colloids may be arranged in diflerent combinations. Certain organic compounds, for example, certain dyestufls such as benzo-purpurin, or Congo red, or similar long compounds of high molecular weight form molecules coming within the molecular size range, that is, between 1 milli-micron (a to about 0.5 micron (1 or more than one-millionth to one-thousandth of a millimeter in dimension, may therefore be used as colloids in my process in conjunction with a colloidal dispersion of opposite charge.

The materials of opposite charge may be dispersed to colloidal condition in any suitable manner as is well known in the art.

Solvents that are miscible with water may be added to aid in the distribution or dispersion of the colloids, especially the resins therein, and to assist their dispersion at the cathode. For example, a mixture of 10% acetone in alcohol may be added to the bath. Other solvents, such as dioxan, may be added or used in the dispersion of the material. These added solvents serve to stabilize, or to increase the stability, of the colloidally dispersed material, and are necessary or advantageous-only with certain dispersions. The solvent will be selected with a view to the nature of the dispersion. For example, in a dispersion of wood tar, the acetone may be omitted.

In the electro-deposition of metals which commonly have more than one valence as, for example, in the case of tin which occurs as bivalent or stannous and quadrivalent or stannlc salts, the eletrolysis should be from a solution containing substantially only the stannous ions.

Electro-deposition from a stannic solution forms a coarser, more roughly. crystalline deposit. while electro-deposition from a solution containing a mixture 01' stannic and stannous ions generally results in a more spongy deposition.

To obtain dense, ilnely grained and lustrous or glossy deposits of tin, electrolysis should therefore take place from a solution which contains a small or negligible quantity, or no, stannic ions and should be under such conditions as to avoid or prevent the formation of stannic ions.

The formation of stannic ions in the electrolytic bath may be avoided or prevented by using tin salts or organic sulphonates and avoiding the tin salts of the usual inorganic or mineral acids. The aromatic sulphonates are the ones useful for this purpose, as they have a much to the required concentration or, a solution of sulphonic acid may be added to a solution of a tin sulphate and, if the concentration of inorganic acid is too high, it may be lowered by the addition of a barium sulphonate or other suitable barium compound or, a suitable tin sulphonate may be obtained by admixing solutions of a stannous sulphate and barium sulphonate in proper proportions. The resulting barium sulphate may be separated by-filtration or decantation, etc. Or, tin may be dissolved directly in a suitable aromatic sulphonic acid.

In making up a bath for electroplating tin, therefore, an organic sulphonate is commonly used. An example of such a sulphonate would be an aromatic sulphonate of tin, such as tin cresol sulphonate or tin phenol sulphonate, which are common aromatic sulphonates. The concentration of the tin salts may vary. Concentrations of less than 10% (for example, about 45% of tin as cresol sulphonate or phenol sulphonate) are generally employed. Such a solution will also contain the positively and negatively charged colloids as, for example, dispersed phenolic resins and glue; for example, as colloidal aggregates. Electrolysis conducted at a density of from 2 to 6 amperes per square decimeter will give a bright or lustrous, fine grained, adherent, deposit of tin.

For other metals, analogous means may be employed in the preparation of suitable electrolytes.

It will be understood that, although the presence of inorganic anions or inorganic acids or salts is undesirable from the standpoint of producing'deposits of high gloss, a small amount as, for example, 4% or 5% of stannous sulphate may not be objectionable.

It will be understood also that any suitable means for colloidally dispersing the dispersed materials may be employed. In the case of glues, gelatins, etc., the dispersion takes place readily by merely soaking the material in water. In the case of resins, tars, etc., dispersion may take place by means of a common solvent in which the resin or tar may be dissolved and then poured into the water with vigorous stirring.

- aqueous medium as,

Other methods that have heretofore been employed for colloidally dispersing materials in an for example, electrolytic methods, formation of insoluble materials in situ in minute dispersion in the dispersing medium as, for example, forming the resin from resiniorming ingredients and diluting, etc., may be employed. These exampleslof the various methods of colloidally dispersing materials. The presence of the colloidally dispersed materials of opposite charge in the acidic electroplating bath is evidenced by an increased hydrogen overvoltage over that'obtained by one of the colloids alone, particularly at low current densities. This is apparent from Fig. 1 of the drawing. 1

Representative examples of electrolytic baths using my invention are as follows: I

methods are, however, merely H from about 3 to 6 amperes per square decimeter in a'still bath. In an higher current densities may be employed. These deposits have ailne grain which cannot be detected under the finest microscope and must, therefore, be below the wavelength of light, thatis less than 0.4 micron Not only are the crystals of which theelectro-deposit is formed of --very small size or dimension, but their internal structure is modified by the introduction in the crystal lattice or unit of a metal compound. In the deposit obtained by my invention, however, one or more of these atomsare replaced by anion-like bodies, for instance, sub-oxides, hydroxides, oxides, etc., of the metal. This replacement of an atom of the metal by a molecule'of one of its compounds,

or of several atoms by the same number of said molecules, causes an enlargement of the crystal Example I Tin in the form of stannous salt .gms 25 Sulphuric acid of 66 Baum gms '75 Glue .-gm 1 Phenol resin gms 1.5 Alpha-naphthol gm 0.5 Water cc 1000 Example II Tin in the form of tin benzene disul- I phonate gms 30 Benzene disulphonic acid gms Gelatin gm 0.8 Beech wood tar cc 0.5 Water cc 1000 Example III Sn as stannous salt gms 25 Naphthalenedisulphonic acid gms Glue 1 Benzoin resin gm 0.8 Alcohol cc 3 Water cc 1000 Example IV Benzoldisulphonate of zinc gms 220 Glue gm- 0.5 Licorice root extract cc 10-20 Beechwood tar cc 1 Water cc 1000 pH 2-5 Example V Cadmium perchlorate gms 250 Agar-agar gm 0 Benzo-purpurine .gm... 1 Water cc 1000 pH 2-5 Example VI 7 Copper phenolsulphonate gms.. 200 Copper perchlorate gms 50 Perchloric acid gms 40 Latex gm Acetone gms 1.5 Gelatine gm 0.1 Water cc 1000 I Example VII Lead perchlorate gms 250 Cresylsulphonic acid gms 80 Extract of caragheen moss cc l0 Beechwood tar gm 1 Water cc 1000 In electrolysing the above baths, metal deposits are obtained using a current lattice and, therefore, a change in the axial ratio. If, for instance, the axial'ratio 'bei'ore introduction of said molecule or molecules was a:b:c, now one of the axes has become elongated and the ratio is now a:b:(c+a:) Through this change new physical and chemical properties are imparted to the metal as, for instance, high gloss. Each of the crystals thus formed will have dimensions less than the wave length of light, that is, less than about 0.4 micron. These crystals are not only of small size, but they have what is known as a preferred orientation, that is, they are all orientated in the same direction. As distinguished from random orientation, crystals orientated in the same direction have an axis of each crystal in substantial parallelism or alignment with corresponding axes of the other crystals and this brings corresponding faces of the crystals into parallelism. For example, X-ray examination of plane sheet products of this invention have shown corresponding axes of each of the crystals to be at substantially a right angle to the surface of the deposit. These various structures are not detectable by the use of a microscope and the ordinary light rays, but may be determined by the use of X-rays which have a much smaller wave length than the ordinary light rays. I

The deposit may also be recognized by a visual inspection because of its very high gloss and because its crystal structure is not detectable with a microscope. In contradistinction to formerly known deposits, the deposits of my invention have a uniform high gloss surface. Where the gloss is too high for commercial use, it may be reduced by brushing with brushes of soft wire, for example, brushes made of German silver wire, in order to reduce the gloss to any desired commercial standard.

Other addition agents besides the colloids may beadded as, for example, agents which lower the surface tension of water, known commonly as capillary active substances, for example, lauric acid, certain amines and phenol, such as alpha-naphthol, given in Example I, or sulphonated oils as, for example, Turkey red oil. Such materials may be added either during the dispersion of the colloids, or afterwards.

What. I claim is:

l. A process of electro-plating tin which comprises passing an electric currentfrom a tin anode densi y 01 75 electro-positive colloidal aggregates, and conagitated bath very much ducting said electrolysis at a current density of from 2 to 6 amperes per square decimeter. A

2. A process of electroplating tin which comprises passing a current from a tin anode to a cathode to be plated through an aqueous plating bath containing a solution of stannous salts, said salts being predominantly organic sulphonates of tin, an acid and colloidally dispersed aggregates electropositive to said solution said aggregates comprising a dispersed material of the group consisting of the phenolic and cresylic resins, bitumens, wood tars, extract from sulphite lignose with sulfonic acids, and cellulose derivatives, electro-positive to said solution and material electronegative to said solution adsorbed on said electropositively charged particles and maintaining a cathode current density upward of 2 amperes per square decimeter. 1

3. A process of electroplating tin which comprises passing an electroplating current from a tin anode to a cathode to be electroplated through an aqueoussolution of an aromatic sulphonate of tin, said solution containing an acid,

colloidally dispersed aggregates electro-positive to said solution and comprising material 01 the group consisting of the phenolic and cresylic rea- ;ins, bitumens, wood tars, extract from sulphite iignose-with sulfonic acids, and cellulose derivatives, electropositive to said solution and material electronegative ,to said solution adsorbed on said eiectropositively charged material and comprising a member of the group consisting of glue, gelatin, agar-agar and albuminous materials, and maintaining a cathode current density upward of 2 amperes per square-decimeter.

4. A base metal having a bright or glossy electrodeposit of tin the crystals of which have a size approximating the wave length of light and are all orientated in the same direction.

5. A bright or glossy electro-deposit of tin having a crystal size approximating the wave length of light, said deposit having a crystal lattice, said crystal lattice comprising tin atoms and a small quantity of a compound of the group consisting of tin hydroxide, tin oxide and tin suboxide and the crystals of said deposit being all orientated in the same direction. 0 MAX SCI-ILOT'IER. 

